A Multiple-Input Multiple-Output (MIMO) system refers to a system with multiple antennas installed at both a transmitting terminal and a receiving terminal. A gain of array processing and a gain of diversity can be further achieved by introducing processing in the space domain to the MIMO system in addition to traditional processing in the time and frequency domains. In the MIMO system, if the transmitter can obtain information about a channel somehow, then it can optimize a transmission signal in view of a characteristic of the channel to thereby improve the quality of reception and lower the requirement of complexity on a receiver. Linear pre-coding/beam-forming as one of optimization schemes is a working approach to deal with the fading channel lower the error probability and improve the performance of the system.
With multiple-antenna linearly pre-coded/beam-formed transmission, information about a channel from a base station to a UE is one of important factors influencing the performance of the system. In a Frequency Division Duplex (FDD) system, the UE feeds the estimated information about the channel back to the base station over the uplink channel by consuming a significant resource of the uplink channel and possibly introducing a quantization error, etc. In a Time Division Duplex (TDD) system, uplink and downlink signals are transmitted in the same frequency band and therefore reciprocity of the uplink and downlink channels holds. The so-called reciprocity refers to that the uplink channel is the same as the downlink channel. The uplink channel can be estimated from the uplink signal transmitted by the UE due to the reciprocity of the uplink and downlink channels to thereby obtain information about the downlink channel while saving a significant portion of feedback overhead.
The reciprocity of channels applies to physical channels propagating in space. A signal needs to be transmitted by a transmitter circuit to an antenna after being processed at baseband, and a signal received from the antenna also needs to be sent by a receiver circuit to the baseband. Generally the transmitter circuit and the receiver circuit are two different circuits, so temporal delays and amplitude gains introduced by the transmitter circuit and the receiver circuit may be different, that is, the transmitter and receiver circuits may not match with each other. The reciprocity of uplink and downlink channels may not be strictly applicable due to the mismatch between the transmitter circuit and the receiver circuit.
A method of cancelling out an influence arising from the mismatch between uplink and downlink circuits is antenna calibration in which a calibration factor is calculated from information reported by the UE and/or information measured by the base station and the channel estimated from the uplink signal is compensated for and adjusted or data to be transmitted is compensated for and adjusted.
Coordinated Multipoint Transmission/Reception (CoMP) refers to coordination between multiple base stations separated in geographical location. The multiple base stations are base stations in different cells or multiple separate bases stations in one cell. Coordinated multipoint transmission/reception is categorized into coordinated downlink transmission and joint uplink reception. Coordinated multipoint downlink transmission is generally categorized into coordinated scheduling and joint transmission. Coordinated scheduling refers to the coordination of time, frequency and space resources between the cells to thereby avoid and lower their interference to each other. Inter-cell interference is a predominant factor restricting the performance of a UE at the edge of a cell, so coordinated scheduling can improve the performance of the UE at the edge of the cell by lowering inter-cell interference. As illustrated in FIG. 1A, coordinated scheduling of three cells can schedule three UEs with possible interference to each other onto resources orthogonal to each other to thereby avoid inter-cell interference effectively.
In a joint transmission scheme, data is transmitted concurrently by multiple cells to a UE to thereby enhance the reception of signals by the UE. As illustrated in FIG. 1B, data is transmitted by three cells to a UE over the same resource, and the signals of the multiple cells are received concurrently by the UE. The superimposition of the useful signals from the multiple cells can improve the quality of the received signals by the UE on one hand and lower the interference to the UE on the other hand to thereby improve the performance of the system.
In analogy to single-cell multiple-antenna transmission, whether coordinated multipoint transmission/reception can be performed effectively depends upon channel state information that can be obtained at a transmitting terminal. The quality of signals can be improved and inter-cell interference can be suppressed by linear pre-coding (i.e., beam-forming) at the transmitting terminal after obtaining the ideal channel state information. The transmitting terminal can obtain the channel state information from a feedback of a user equipment, but the channel feedback may consume a valuable uplink spectrum resource to thereby lower the uplink spectrum efficiency, which may be particularly apparent in coordinated multipoint transmission/reception because each base station participating in coordinated transmission needs to obtain the channel state information of the user equipment, so there may be a feedback overhead increasing linearly with the number of coordinating base stations. Required precision of the channel state information may also be higher with a particular transmission scheme, which will mean a larger uplink bandwidth resource to be occupied. Also the channel state information may be fed back inevitably with a quantization error due to a limited capacity of the uplink channel. The quantization error may lower the performance of coordinated multipoint transmission/reception. The channel state information can be obtained in the TDD system due to the reciprocity of channels as a very competitive solution without incurring any extra feedback overhead and without introducing any quantization error due to the feedback. The CoMP solution based on the reciprocity of channels may also be faced with required antenna calibration.
Uplink and downlink antenna calibration of the same base station can be well performed by self-calibration in the practical system, but there has been no working approach to address antenna calibration between base stations, so that the uplink and downlink reciprocity of joint channels of the multiple base stations may not be strictly applicable. Without the satisfactory uplink and downlink reciprocity, downlink joint channels of the multiple base stations cannot be obtained based upon a measured uplink channel in the TDD system, thus failing to make use of the advantage of the TDD system.
In summary, the user equipment involved in existing calibration method over the air interface is required to feed back channel coefficients which have to be quantized directly with a significant uplink overhead, thus degrading the efficiency of the system.